。
Let's play tennis after school.
Literal
After-school [at-に] tennis [object-を] do-[volitional-polite-ましょう]
The polite volitional ~ましょう ('let's do X') turns する into しましょう, proposing an activity to the listener at desu/masu register — perfect for classmates or teammates suggesting a plan. Plain-form しよう would do the same job at a casual register. 放課後 ('after school') is a fixed compound used by students of all ages; に marks the time at which the action happens.